Conventionally known hydraulic transmissions include pure hydraulic transmissions (hydrostatic transmissions referred to as HSTs) which convert all input power from an engine into oil pressure for transmission and hydro-mechanical (power-split type) transmissions (HMTs) which hydraulically transmit part of the input power while mechanically transmitting the rest of the input power. Since the latter transmissions (HMTs), which convert only part of mechanical power into hydraulic power, transmit the mechanical power with high efficiency, they have the advantage of achieving higher efficiency than the former transmissions (HSTs). Accordingly, the HMTs are said to be ideal transmissions for vehicles subjected to significant load changes, such as bulldozers and wheel loaders, and some of them are adopted in such vehicles.
A typical hydro-mechanical transmission (HMT) achieves variable speed characteristics by means of a planetary gear mechanism, or more particularly, by the following arrangement. Of three elements of the planetary gear mechanism (i.e., a sun gear, a carrier provided with planetary gears and a ring gear), a first element is coupled to an input shaft, a second element is coupled to an output shaft, and a third element is coupled to a hydraulic pump or hydraulic motor. Rotational speed of the hydraulic pump or motor is varied, thereby changing rotational speed of the output shaft.
There are two types of HMTs. One is an “output-split type” in which the pump/motor fluidly connected by a hydraulic circuit to another pump/motor coupled to the planetary gear mechanism is coupled to the input shaft of the transmission at a constant speed ratio. The other is an “input-split type” in which the hydraulic pump or hydraulic motor fluidly connected by the hydraulic circuit to another hydraulic pump or hydraulic motor coupled to the planetary gear mechanism is coupled to the output shaft of the transmission at a constant speed ratio.
There is an electro-mechanical transmission (EMT), a technique similar to the HMT. In place of the pump/motor used in the HMT, this EMT uses a generator/motor for converting part of the mechanical power into electric power for transmission. A prior art relating to this EMT is disclosed in patent document 1. The transmission disclosed in this document is an electro-mechanical transmission having two planetary gear mechanisms and two electric motors and is configured to perform shifting with clutches to establish an input-split mode at low speeds and a compound-split (output-split) mode at high speeds.
Here, patent document 1 is U.S. Pat. No. 6,478,705.